This paper describes the concepts and implementation of a design aid for the internal schema of an existing CODASYL-like database system. It allows for tailoring the storage structure level to a given logical schema and a specified workload. According to the 1978 CODASYL report, our DBMS provides two levels of schema declaration, the DDL-level for logical schema description and a DSDL-like level for specifying the storage structures to implement the objects of the logical schema. The repertoire of storage structures supported by our system is briefly sketched. The performance predictions our tool uses for finding a good internal schema are basically heuristic. This approach is justified by weighing its advantages and shortcomings against those of analytic models and simulation. Finally, some preliminary user experiences with a pilot version are related. © 1984 IEEE.